Plasmonic self-assembled colloidal magnetic resonators, or clusters of metallic colloidal nanoparticles that support magnetic dipole resonances at near-infrared frequencies, have been demonstrated in this study. Each cluster, or trimer, consists of three gold nanospheres coated with polymer that have undergone self-assembly from solution. The resulting structures have separation gaps between the particles of about two nanometers, which surpasses the resolution obtainable through traditional approaches such as lithography. This result demonstrates that self-assembly of colloids offers a promising route to low cost and complex three dimensional metamaterials.

Summary

Figure 1. Diagram of the self-assembly of metallic colloidal particles into a trimer, with the LC circuit analogy.

Researchers are interested in colloidal metallic nanostructures because they can support surface plasmons, or oscillations of free electrons in the metal that couple with the electromagnetic field. The plasmonic properties of nanostructures depend strongly on their geometric and material properties. Precision at the nanometer scale is necessary to tune the resonant frequencies supported by these structures and self-assembly of the colloidal particles is one way that this can be achieved. Here the authors have assembled trimers of three dielectric-coated gold nanoparticles with controllable gap separations and measured their electromagnetic properties with correlated scattering spectroscopy.

The magnetic resonant behavior of these structures makes sense when one considers the notion that the structure is analogous to an LC circuit (see Figure 1) consisting of nanocapacitors (the separation gaps) and nanoinductors (the metal nanoparticles themselves, which individually support a plasmon resonance). To build these structures, gold nanoshells are first synthesized around silica cores. The plasmon resonance can be tuned by varying the ratio of the core and shell radii. A dielectric coating defines the gap separation in the clusters.

These polymer-coated nanoshells are then assembled by placing a droplet of solution onto a hydrophobic substrate and allowing it to dry at room temperature. The droplet separates into smaller droplets as it dries, some of which contain three nanoshells, which in the end will assemble into a trimer because surface tension causes them to remain enclosed inside the droplet. Thus, the trimer is held together by Van der Waals forces once it forms and is the entropically favored arrangement for the three particles. The optical properties of the structures may then be probed using dark field spectroscopy.

Figure 2. Electrostatic simulation of a trimer. (A) Resonant wavelengths for different modes as a function of gap separation. (B) Electric potential (coloring) and displacement currents (vector field) of the first order electrostatic modes. The magnetic dipole mode supports a circulating displacement current.

It is possible to solve for the electrostatic modes of the trimer structure using a numerical finite element method, and these can be used for comparison with and interpretation of the spectral data. The resonant mode energies can be predicted as a function of gap separation with this technique, and one can model the structures to understand the magnetic and electric dipole resonance modes (see Figure 2). With decreasing gap separation, the capacitive coupling increases, causing all the resonant energies to decrease.

Scattering spectra can be compared with predictions based on these simulations. There is strong agreement between the data and simulations except for some effects in the data that can be attributed to geometric perturbations due to asymmetries at the trimer gaps. In order to mitigate these effects, higher quality spherical particles or larger gap sizes are necessary.

Self-assembly has high potential as a tool for engineering plasmonic nanoclusters out of colloids. DNA or emulsion technology could be used as a means of tuning the resulting structures, and three dimensional clusters with tetrahedral or higher polyhedral symmetry could be generated. Nonspherical particles or clusters consisting of particles of different sizes could also be used to explore more general plasmonic resonance behavior. These systems offer promising routes to negative refraction in the near-infrared or visible ranges and other novel metamaterial properties.

Connection to soft matter

While the electromagnetic properties of this system are particularly interesting, principles from soft matter are quintessential in the process by which the authors were able to assemble these structures. First and foremost, the process is an important example of self-assembly of a colloidal system. By placing a droplet of the solution on a hydrophobic surface and allowing it to dry, colloidal aggregation of small clusters of the nanoparticles is entropically favored. This occurs because surface tension at the liquid-air interface causes the particles to remain enclosed within the droplet, while the high contact angle between the droplet and the substrate prevents aggregation of the nanoparticles at the substrate interface.

Self-assembly and soft lithographic techniques from soft matter are key to the assembly of these structures, and could be used to create more complex three-dimensional colloidal aggregates in controllable and reversible ways. The potential applications of these metamaterial systems motivate further experimentation and development of these self-assembly techniques, and encourage deeper understanding of the underlying soft matter physics.